The object of the present invention is a process for the preparation of vinyl chloride homopolymers and copolymers in the form of a powder suitable for yielding platisols with improved rheological properties. The vinyl chloride homopolymers and copolymers prepared are also the object of the invention.
It is known that for certain applications in which the vinyl chloride polymers are used in the form of plastisols such as, for example, coating, it is necessary to use polymers in the form of a powder suitable for yielding plastisols which, in the presence of very low amount of plasticizer; as low as 25 parts by weight of plasticizer per 100 parts by weight of polymer, present the lowest possible viscosity both at low rate gradient and high rate gradient.
Vinyl chloride homopolymers and copolymers suitable for yielding plastisols are prepared by homopolymerization of the vinyl chloride in emulsion or in micorsuspension, or copolymerization of the vinyl chloride in emulsion or in microsuspension with another copolymerizable monomer. This homopolymerization or copolymerization industrially provides a latex generally containing 30 to 60% by weight of dry material formed by vinyl chloride homopolymer or copolymer particles having a diameter generally between 0.1 to 5.mu.m. To obtain a dry pulverulent polymer, the said latex, possibly following concentration, for example, by ultrafiltration, is generally treated by drying, especially by drying by atomization.
The drying of the said latices by atomization is a well-known technique which is carried out in an apparatus consisting of an atomization device fed with the latex and an atomization chamber in which the said device continuously disperses the latex in fine droplets. In this device, hot gases, generally air, come into contact with the pulverized mass and evaporate the water present in each droplet. The temperature of the gases entering the atomization chamber is generally between 170.degree. and 260.degree. C., and their temperature upon leaving the chamber is between 65.degree. and 80.degree. C. From the latex particles, polymer agglomerates having a diameter generally between 30 and 150.mu.m are thus continuously formed, separated from the hot gases and then finally subjected to, also continuously, to grinding intended to lead to sizes generally between 5 and 30.mu.m. The grinding can especially be carried out by means of a device using mechanical energy or compressed air to precipitate the agglomerates against a solid wall and/or against one another, thus reducing their size.
According to the processes currently used, especially for reasons pertaining to the homogeneity of the polymer to be prepared, the vinyl chloride homopolymer and copolymer latices produced according to successive polymerization operations are mixed in a storage tank where they are kept at ambient temperature and, at this ambient temperature, they are subjected to the treatment of drying by atomization.
The resultant pulverulent powders do not produce plastisols having the desired rheological properties and still contain undesired levels of vinyl chloride monomer in the polymer.